The present invention relates generally to a thermally-activated bimorph optical switching element. More particularly, the invention relates to an optical switching system for fiber optic cross connect and add-drop multiplexers in wavelength division multiplexing systems.
The growth of the Internet and the World Wide Web has created a demand for large data bandwidths and high-speed channel switching within the telecommunications industry. Traditionally, telecommunications and other data transfer signals have been exchanged electrically via electrically-conductive wires and cables. Increased bandwidth requirements have led to the adoption of optical signal transfer via fiber optic cables. Optical fiber communications were originally developed for large bandwidth applications, such as long distance trunk cables connecting local metropolitan telephone exchanges. As the cost of these fiber optic networks has decreased and the demand for signal bandwidth increased, fiber optic communications networks are being installed in local metropolitan area networks, and as parts of local area networks (LANS) for data exchange between computers in offices and other commercial and government environments.
Crucial elements in these fiber optic communications networks are the switches and routers that direct the optical signals from various sources to any one of a multitude of destinations. Such switches can be used in a variety of applications, including Nxc3x97N cross-connect switches for switching signals between arrays of input and output optical fibers, add-drop multiplexers in wavelength-division-multiplexing (WDM) and the more recent dense-wavelength-division-multiplexing (DWDM) systems, reconfigurable networks, and hot backups for vulnerable components and systems. These and other similar applications require switches with moderate switching speeds of one millisecond or less, low insertion loss, low noise, low dispersion, and low cross talk. Additional needs are for low cost, small form factor, easily manufacturable components to facilitate the rapid adoption of these technologies in wider arrays of optical switching applications.
Many switching techniques have been developed to facilitate the generation and transfer of these optical signals in fiber optic networks. Initially, to switch signals between various arrays of optical fibers, the optical signals had to be converted to electrical signals, conditioned, amplified, and routed to laser light sources and modulators before being retransmitted to the output optical fiber. Electrical switching schemes are undesirable in these applications due to the reduced bandwidth, increased noise, and added cost and complexity of the resulting system designs. As a result, several all-optical switching schemes have been developed, or are under development, for use in telecommunications and data transfer networks, but all of these designs have limitations in these applications.
What is needed, therefore, is a reliable, low-cost, high-bandwidth, low-loss optical switching system.
The foregoing and other needs are met by a device for switching optical signals, completely within optical media, from an arbitrary number of N input optical fibers to a different set of M output optical fibers. The invention uses a thermal bimorph optical switch to redirect optical radiation emitted by a laser light source, or the end of an optical fiber, to an input end of another optical fiber. The modulated optical radiation containing signals from the input fiber optic bundle or laser light source is collimated into parallel beams and projected in free space across the tops of an array of microcantilever bimorph optical switches. When selected, a particular switch is activated, pops up, intercepts, and reflects the optical radiation down into a cavity and then into a short section of parallel waveguide. The radiation is directed into a transverse waveguide and then to the output optical coupler. This radiation is then coupled into the selected output optical fiber.
The microcantilever (or bimorph) optical switch is composed of two or more thin film layers that possess large differences in their thermal expansion coefficients. When heated, the difference in thermal expansion of the two layers induces a surface stress on the cantilever structure, causing it to bend to null the resultant stress. The bending of the cantilever structure causes the free end of the cantilever to move to intercept the optical radiation emitted by the laser light source or optical fiber.
The bottom layer of the bimorph switch is fabricated from a metal that is highly reflective at the wavelength of the optical radiation, and that possesses a large thermal expansion coefficient. A thin dielectric electrically-insulating middle layer is usually added to the structure to electrically isolate the bottom layer from the top layer. The upper layer is fabricated from a low thermal expansion, doped semiconductor material, which is patterned and doped to create a resistive heater that is used to uniformly heat the bimorph structure.
When a switch is selected, an electrical current passes through the resistive heater, thereby heating the bimorph structure. When the structure is heated, the free end of the cantilever structure rises and intercepts the beam. The optical radiation is reflected by the metal surface into the free space cavity, through a transparent window, and into the waveguide material. The optical radiation is directed into a short length of coupling waveguide structure, and turned approximately 90xc2x0 into a Y coupler and directed into another waveguide structure approximately at 90xc2x0 to the original direction of beam propagation. The waveguides are then coupled into another set of output optical fibers.
The optical switch of the present invention offers the following benefits: (1) high speed switching with thermal time constants of less than 1 millisecond, (2) true Nxc3x97M non-blocking switching operation, (3) scalability to large switch arrays where N and M may be several hundred to a thousand switch array dimensions, (4) true digital switching operation; i.e. on/off with no precise control of switch paddle position required, (5) in-plane switch construction with resultant ease of alignment, (6) low insertion losses and cross talk, (7) small physical size, and (8) low cost fabrication due to complete silicon integrated circuit compatibility.
In one aspect, the invention provides an optical switch for selectively transferring optical radiation from a first optical path to a second optical path. The switch includes an input optical structure for receiving the optical radiation from the first optical path and for directing the optical radiation along a propagation path. The switch also includes a paddle disposed adjacent the propagation path. The paddle has a fixed portion and a free end, and is operable to bend upon a change in temperature, thereby moving the free end between a first position where the free end does not intercept the optical radiation and a second position where the free end intercepts and redirects the optical radiation. The switch further includes an output optical structure for receiving the optical radiation from the paddle and for directing the optical radiation along the second optical path.
In another aspect, the invention provides an optical switching device for selectively transferring at least one optical signal from at least one optical input to M number of output optical channels. The switching device includes an input optical structure for receiving the optical signal from the optical input and for directing the optical signal along a propagation path. The switching device also includes a 1xc3x97M-dimensional array of optical switches. The 1xc3x97M-dimensional arrays have M number of selectively-activated paddles that are adjacent to and in parallel with the propagation path. Each of the paddles has a fixed portion and a free end, and is operable to bend upon a change in temperature, thereby moving the free end between a first position where the free end does not intercept the optical signal and a second position where the free end intercepts the optical signal. One paddle at a time is selectively activated to bend, intercept the optical signal, and redirect the optical signal. The switching device further includes M number of output optical structures, each optically coupled to a corresponding one of the M number of paddles. Each of the output optical structures receives an optical signal from a corresponding one of the paddles and directs the optical signal along a corresponding one of the M number of output optical channels.